Information on size and velocity of spherical objects including particles, droplets, bubbles, etc., is important for numerous applications in various industries. These applications include, for example, fuel spray combustion analysis and control for the automotive industry, aircraft gas turbine combustion, inhaler manufacturing for the pharmaceutical industry, household spray systems manufacturing, agricultural pesticide application, aircraft icing analysis and control, spray nozzle manufacturing, atmospheric aerosol analysis, atmospheric studies, and various combustion related applications.
Normally, a laser light scattering interferometry technique is used to determine the size and velocity of spherical objects, such as particles, drops, bubbles, etc. According to this technique, spherical objects pass the intersection point of two crossed laser beams generated from the same laser. The two crossed laser beams form a sample volume at the intersection point. The light scattered by the spherical object, as it passes through the sample volume, produces an interference fringe pattern at the plane of the detector. The spatial period of the interference fringe pattern produced by the light scattered from the spherical object, as it passes through the sample volume, may be used to determine the size of the spherical object and a velocity component of the spherical object.
The light produced by each of the two crossing laser beams is scattered from the spherical object due to various mechanisms, e.g., reflection and refraction. The light scattered from the spherical object by different mechanisms that cannot be reliably separated is mixed to form the interference fringe pattern that is a complex superposition of several interference patterns having several spatial frequency components. Furthermore, if more than one particle passes or resides in the sample volume at one time, there may be a change in the signal frequency and phase of the signals leading to an error in the measurement.
Such complex interference fringe patterns deviate significantly from a sinusoidal fringe pattern formed by the light scattered from single spherical object due to a single light scattering mechanism. The complex interference fringe pattern formed by interference between the different light scattering mechanisms, e.g., refraction and reflection, varies in time and space as the spherical object moves through the sample volume. The complex non-periodic interference pattern with varying spatial period leads to significant errors in determining the size and velocity of the spherical object. The problem is intensified in high particle density environments, when highly focused laser beams having Gaussian beam intensity distributions are used.
FIG. 1 illustrates drops having different relative diameters to the focused Gaussian beam diameter that pass Gaussian laser beams 110-130. As shown in FIG. 1, each of the laser beam intensity profiles 112, 122, and 132 has a waist diameter Dw. A particle 111 has the diameter d smaller than diameter Dw, such that Γ=d/Dw is less than 1, a particle 121 has the diameter d of the same order of the magnitude as diameter Dw, such that Γ=d/Dw is approximately equal 1, and particle 131 has the diameter d larger than diameter Dw, such that Γ=d/Dw is larger than 1, as shown in FIG. 1. As shown in FIG. 1, for particle 111 having Γ=d/Dw<<1 that is approximately uniformly illuminated, the light intensity of the refracted light 113 is greater than that of the reflected light 114. For particle 131, having Γ=d/Dw>>1, and passing on certain trajectories through the Gaussian intensity beam, the light scattering efficiency for either of the refracted light 133 or the reflected light 134 is dominant, as shown in FIG. 1. For particle 121, having Γ=d/Dw˜1, the light intensity of the refracted light 123 is the same order of the magnitude as that of the reflected light 124, as shown in FIG. 1. Particle 121 is non-uniformly illuminated because of the significant difference in the incident intensities of the laser beam at points 125 and 126 of particle 121, as shown in FIG. 1. The non-uniform illumination of particle 121 by a laser beam having a Gaussian intensity profile 122, as shown in FIG. 1, causes the intensities of the reflected and refracted components of the scattered light to be comparable in magnitude for particles passing on certain trajectories through the beams. As shown in FIG. 1, for the particles 121 having diameter d that is the same order of the magnitude as a focused Gaussian laser beam diameter Dw, the light scattered by reflection and the light scattered by refraction have similar intensities when passing on certain trajectories through the beams. The light scattered by reflection and refraction components having similar intensities forms a complex interference fringe pattern and can produce a progressively increasing magnitude of measurement error.
Thus, the Gaussian intensity profiles of the incident laser beams and the random trajectories of particles through the crossing laser beams can cause the intensities of the light scattered from the spherical objects due to different light scattering mechanisms, e.g., reflection and refraction, to be of similar order of magnitude. Additionally, because the sign of the phase shift for the interference fringe pattern produced by reflected light is opposite to that produced by refracted light, the fringes produced by reflection appears to move in the opposite direction to the fringes produced by refraction. The light scattered from the spherical object by the various mechanisms, e.g., refraction and reflection, mixes to produce the interference fringe pattern that is complex and not spatially periodic.
The simultaneous detection of more than one of the light scattering components mixed together limits the measurement resolution and leads to significant errors in determining the size and velocity of the spherical objects. The problem becomes more serious as the diameter of the spherical object approaches the diameter of the focused laser beam. In such cases, the size of the particles passing on certain trajectories through the beams may be grossly overestimated. That is, complex non-periodic interference pattern formed by the mixed light scattered from the spherical object due to various scattering mechanisms leads to significant errors in measuring the size and velocity of the spherical object that severely impacts the device performance. More than one spherical object passing the measurement volume at one time may lead to significant error in measuring the size and velocity of the spherical object. These problems are related since a larger sample volume will lead to a great probability of having more than one object in the sample volume at one time. Decreasing the size of the measurement volume to reduce the occurrence of these coincident events leads to the trajectory problem when using Gaussian beam intensity profiles.